Got some time on the water to break my motor in since rebuild. Everything is going good but towards …

Phil Caputo asks: Got some time on the water to break my motor in since rebuild. Everything is going good but towards the end of the day I couldn’t get past 3500 rpm. It felt like it was missing or breaking up. I’m thinking plugs are fowled out. Motor is a 98 efi 200 merc I’m running 25-1 oil mixture